False dilemma is a logical fallacy in which two options are presented as if they are mutually exclusive or as if they're the only possible options, when in fact they are not. [1] It is used to give the impression that if one rejects another's argument, he must necessarily embrace an absurd or unappealing stance.
An example of a false dilemma is the phrase "You're either with us, or you're against us." It is a false dilemma because it is possible that you are completely unaware or apathetic towards the issue.
Of course, not all dilemmas are false dilemmas.